On the determination of the diffusion coefficient of ionic species through porous materials

The knowledge of the diffusion coefficient of ionic species through porous materials is an essential input parameter for modelling transfer phenomena, for predicting material behaviour on long periods and for assuring the confinement of radioactive materials. In this paper we propose to develop an experimental way to obtain the diffusion coefficient of chloride through porous materials. The method, based on the application of the Nernst-Einstein equation, requires the determination of the electrical conductivity of the specimen. On that purpose, an electrochemical impedance spectroscopy (EIS) test is used. An experimental set-up has been established and carefully controlled in order to improve the accuracy of the measurements. The resulting impedance spectra are analyzed by means of two different electrical models. A conventional equivalent circuit is first applied to get the main spectrum- shape features and the material characteristics. Another physical phenomenon-based equivalent circuit is then used and intended to provide better explanation of the material microstructure and its electrical behavior. The results obtained are validated with data previously found, arising from an electro-kinetic test based on the Nernst-Planck equation